1. Field of the Invention
The present invention relates generally to a ball-screw-driven linear actuator for lifting up and down a heavy load, and more particularly to a linear actuator including an anti-reverse-rotation mechanism for preventing an actuating member from moving in a loaded direction due to rotation of a ball screw when rotation of the ball screw is stopped under the condition in which the actuating member reciprocally movable in unison with a ball nut is subject to a thrust load.
2. Description of the Related Art
In a handling work for lifting up and down heavy-weight articles, screw-driven linear actuators are widely used. The screw-driven linear actuators are constructed such that by rotating a screw shaft by means of a rotary drive source such as a reversible motor, an actuating member adapted to be subjected to a load such as the weight of a heavy article is driven to move back and forth in the axial direction of the screw shaft via a nut threaded with the screw shaft.
Many such known screw-driven linear actuators employ a ball screw mechanism which consists of a threaded shaft (ball screw shaft) linked to a threaded nut (ball nut) by balls constrained to roll the space formed by the threads, in order to reduce friction. The ball screw mechanism is advantageous for its capability of achieving a smooth motion-converting operation with reduced drive torque.
Apart from its high transmission efficiency, the linear actuators using the ball screw mechanism have a problem that when the ball nut is subject to a thrust load while rotation of the ball screw shaft is stopped, the thrust load tends to rotate the ball screw, thereby causing the actuating member to move in a direction of load (loaded direction) with the heavy-weight article supported thereon.
To cope with this problem, various improvements have been proposed, such as disclosed in Japanese Patent Laid-open Publication No. HEI 8-322189.
The Japanese publication specified above shows a linear actuator equipped with a mechanism for preventing rotation of a ball screw which would otherwise occur due to the effect of a thrust load. More specifically, the linear actuator, as re-illustrated here in FIG. 5, includes a worm wheel A3 rotatably supported within a housing A1 by means of a ball bearing A2 which is capable of bearing both a radial load and a thrust load. One end (proximal end) of a ball screw shaft A4 is firmly fitted in a central hole (not designated) of the worm wheel A3 for co-rotation therewith. Rotation of an electric motor (not shown) is transmitted from a worm A5 through the worm wheel A3 meshing with the worm A5 to the ball screw shaft A4. Rotation of the ball screw shaft A4 is converted into linear reciprocating motion of an actuating member (not shown) connected to a ball nut (not shown) threaded with a screw portion (not shown) formed on the distal end side of the ball screw shaft A4.
On the ball screw shaft A4, there are assembled two sets of components, each component set including one washer A6 or A7, one sleeve A8 or A9, one roller clutch A10 or All, and one friction plate A12 or A13. The roller clutches A10, All are mounted such that respective free-rotating directions of the roller clutches A10, A11 are opposite to each other.
With this arrangement, one of the roller clutches A10 (A11) operates to lock the associated sleeve A8 (A9) against rotation in one direction, while the other roller clutch A11 (A10) permits rotation of the associated sleeve A9 (A8) in the same direction.
The washers A6, A7 are fixed to the ball screw shaft A4. In the linear actuator, when the ball screw shaft A4 while being stopped is subjected to a thrust load in one direction tending to retract the ball screw shaft (hereinafter referred to as "retracting direction"), the thrust load is born by the ball bearing A2 successively through the washer A6, friction plate A12 and sleeve A8 that are disposed on the right-hand side in FIG. 5.
In this instance, the ball nut (not shown) applies a force tending to rotate the ball screw shaft A4 (hereinafter referred to as "rotating force"). However, since the roller clutch A10 is designed to lock the sleeve A8 against rotation in the acting direction of the rotating force, a braking torque is applied from the sleeve A8 through the friction plate A12 to the washer A6 and thus prevents rotation of the ball screw shaft A4.
When the ball nut is to be advanced against the thrust load of the retracting direction, the ball screw shaft A4 is rotated in the opposite direction to the rotating force whereupon the roller clutch A10 releases the sleeve A8 to thereby allow freewheeling of the sleeve A8.
In this instance, the roller clutch All locks the sleeve A9 against rotation. However, since the ball screw shaft A4 is still subjected to the thrust load of the retracting direction, the washer A7 is not forced against the friction plate A13. Additionally, since the worm wheel A3 is freely rotatable relative to the sleeve A9, no braking torque is produced with respect to the ball screw shaft A4.
When the ball screw shaft A4 is rotated to retract the ball nut in the same direction as the thrust load, the roller clutch A10 locks the sleeve A8 against rotation. Accordingly, the washer A6 is rotating while being subjected to a braking torque applied thereto from the locked sleeve A8 through the friction plate A12.
In the case where the ball screw shaft A4 while being stopped is subjected to a thrust load in the opposite direction tending to extend the ball screw shaft A4 (hereinafter referred to as "extending direction"), the thrust load is born by the ball bearing A2 successively through the washer A7, friction plate A13, sleeve A9 and worm wheel A3 that are disposed on the left-hand side in FIG. 5.
In this instance, a rotating force in the opposite direction to the thrust load of the retracting direction is applied from the ball nut (not shown) to the ball screw shaft A4. However, since the roller clutch A11 locks the sleeve A9 against rotation in the direction of the rotating force, a braking torque is applied from the sleeve A9 through the friction plate A13 to the washer A7 which is secured to the ball screw shaft A4. Thus, rotation of the ball screw shaft A4 is prevented.
When the ball nut is to be retracted against the thrust load of the extending direction, the ball screw shaft A4 is rotated in the opposite direction to the rotating force whereupon the roller clutch A11 releases the sleeve A9 to thereby allow freewheeling of the sleeve A9.
In this instance, the roller clutch A10 locks the sleeve A8 against rotation. However, since the ball screw shaft A4 is still subjected to the thrust load of the extending direction, the washer A6 is not forced against the friction plate A12. Additionally, since the ball screw shaft A4 is freely rotatable relative to a sleeve bearing A14 disposed interiorly of the sleeve A9, no braking torque is produced with respect to the screw the ball screw shaft A4.
When the ball screw shaft A4 is rotated to advance the ball nut in the same direction as the thrust load of the extending direction, the roller clutch A11 locks the sleeve A9 against rotation. Accordingly, the washer A7 is rotating while being subjected to a braking torque applied thereto from the locked sleeve A9 through the friction plate A13.
In the conventional linear actuator shown in FIG. 5, the sleeves and the one-way clutches are used in combination to form two sleeve-and-clutch pairs. Since the sleeve-and-clutch pairs are disposed in series with the ball bearing in the axial direction of the ball screw shaft, the overall axial length of the conventional linear actuator is relatively large. Additionally, due to a relatively large number of parts used, the conventional linear actuator requires a relatively long time for assembly and adjustment which might increase the manufacturing cost of the linear actuator.